When drilling, it is very important that the start-up is performed in a correct manner. Thus, at the start-up of drilling into rocks with a percussion rock drilling machine, it has to be ensured that the first part of the hole is accurately accomplished in order for the hole to be positioned at an intended place and have a correct direction.
In order to obtain a good start of the drilling, it is desired to try to control the drilling steel as good as possible near the drill bit at the beginning of the drilling (drill steel support), as well as to drill the first part of the hole using a reduced feed force and a reduced drilling power in order to prevent the drilling steel from sliding against the surface of the rock. In other words, the critical part of the drilling, i.e. the start-up or so called collaring, should be smooth and careful until there has been formed a deep enough hole having a correct direction, whereafter full feed force and drilling power may be utilized. What constitutes a deep enough depth depends to a large part on the quality of the rock. For example, soft rocks having many cracks may require a deeper hole to ensure a correct direction, before full feed force is used.
When the use of hydraulic rock drilling machines at first started, generally, these were powered by uncomplicated direct controlled hydraulic systems. The collaring was realized by starting the drilling at a reduced hydraulic pressure supply to the percussion circuit and feed drive of the drilling machine. After a certain time, or when a certain drilled hole depth is reached, the pressures were increased to such values that resulted in a desired balance between drilling speed and working life of the drilling steel (full drilling). This pressure increase was performed by adjusting sequence valves, and the pressure increase process depended on the valves and ducts in the hydraulic system.
When more modern electrically and computer controlled hydraulic systems later have come in use, the process of an initial drilling step has been maintained, in which a reduced drilling machine power and feed force is used, and a final drilling step where full power is utilised. A suitable transition occurs there between.
An example of such a previously known method for controlling drilling parameters is shown in the European patent EP 0 564 504. This publication discloses a method for controlling a rock drilling process, and in accordance with the method described therein the percussion force and feed force of a drilling machine are adjusted, so that the rotational power of the drill does not exceed a pre-set limit value.
This is done by controlling the drilling in at least three different stages, of which the first stage constitutes the start-up drilling, the second a transition stage to the third, which in turn is the normal operation. According to the method, suitable values for each drilling stage are to be set, so that the percussion force and feed force are optimal for each stage.
There are several drawbacks with the method described in EP 0 564 504. An obvious drawback is that it is not always known in advance what is optimal for a certain stage, and it is not evident from said document how the predetermined feed force and percussion force values for the respective stage are to be determined in order to be optimal. Another drawback is that the process with three or more stages in the control program is unnecessarily complicated, since, among other things, it has to be determined, on the one hand, how long the first reduced stage is to go on, and, on the other hand, what the transition stage should look like.
The transition step should be smooth, but not unnecessary extended in order to avoid that time is lost because a great part of the hole is drilled at a lower power than the available full power. Consequently, the parameters that have to be set constitutes a considerable drawback of the method shown in EP 0 564 504. There are a number of parameters to adjust for each of the three or more stages, for example, different periods of time, percussion force, feed force, drilling time, drilling depth, speeds etc. Moreover, discontinuities in the direction of drilling parameter increase may give incorrect information to those parts of the automatic control system that supervise these parameters in order to detect a drill possibly getting stuck.
It would thus be desirable to provide a method and an apparatus that simplifies and improves the initial phase of drilling in rock.